Determines whether the arrow cap is filled
#include <GDIPlus.au3>
_GDIPlus_ArrowCapGetFillState ( $hArrowCap )
| $hArrowCap | Handle to a ArrowCap object | 
| True: | Arrow cap is filled. | 
| False: | Arrow cap is not filled or if error then @error flag is set. | 
Search GdipGetAdjustableArrowCapFillState in MSDN Library.
#include <GDIPlus.au3>
#include <GUIConstantsEx.au3>
#include <MsgBoxConstants.au3>
Example()
Func Example()
    Local $hGUI, $hGraphic, $hPen, $hEndCap
    ; Create GUI
    $hGUI = GUICreate("GDI+", 400, 300)
    GUISetState(@SW_SHOW)
    ; Create resources
    _GDIPlus_Startup()
    $hGraphic = _GDIPlus_GraphicsCreateFromHWND($hGUI)
    $hPen = _GDIPlus_PenCreate(0xFF000000, 4)
    $hEndCap = _GDIPlus_ArrowCapCreate(4, 6)
    ; Show fill state
    MsgBox($MB_SYSTEMMODAL, "Information", "Fill state: " & _GDIPlus_ArrowCapGetFillState($hEndCap))
    ; Draw arrow 1
    _GDIPlus_PenSetCustomEndCap($hPen, $hEndCap)
    _GDIPlus_GraphicsDrawLine($hGraphic, 10, 130, 390, 130, $hPen)
    ; Draw arrow 2
    _GDIPlus_ArrowCapSetFillState($hEndCap, False)
    _GDIPlus_PenSetCustomEndCap($hPen, $hEndCap)
    _GDIPlus_GraphicsDrawLine($hGraphic, 10, 160, 390, 160, $hPen)
    ; Loop until the user exits.
    Do
    Until GUIGetMsg() = $GUI_EVENT_CLOSE
    ; Clean up resources
    _GDIPlus_ArrowCapDispose($hEndCap)
    _GDIPlus_PenDispose($hPen)
    _GDIPlus_GraphicsDispose($hGraphic)
    _GDIPlus_Shutdown()
EndFunc   ;==>Example